At present, water-absorbent resins (water-absorbing agents) and hydrophilic fibers (e.g. pulp) are widely used for sanitary materials such as disposable diapers, sanitary napkins, and so-called incontinent pads as their component materials for the purpose of causing the water-absorbent resins and the hydrophilic fibers to absorb body fluids. Examples of materials used as main raw materials for the above water-absorbent resins include: partially-neutralized and crosslinked poly(acrylic acids); hydrolyzed graft polymers of starch-acrylic acid; saponified copolymers of vinyl acetate-acrylic acid ester; hydrolyzed copolymers of acrylonitrile or acrylamide, or crosslinked polymers of these hydrolyzed copolymers; and crosslinked polymers of cationic monomers.
In recent years, as to these sanitary materials such as disposable diapers and sanitary napkins, their high functionalization and thinning are making progress, so there is a tendency toward increases in the amount of the water-absorbent resin as used per piece of sanitary material and in mass % of the water-absorbent resin relative to a whole absorbent structure consisting of such as the water-absorbent resin and the hydrophilic fibers. Specifically, the ratio of the water-absorbent resin in the absorbent structure is raised by decreasing the amount of the hydrophilic fibers (which have a small bulk density) and increasing the amount of the water-absorbent resin (which has excellent water absorbency and a large bulk density) as used. Thereby the thinning of the sanitary materials is aimed at without lowering the water absorption quantity.
(1) Liquid Permeability and Liquid Diffusibility:
However, the sanitary materials, in which the ratio of the hydrophilic fibers has been decreased and that of the water-absorbent resin has been increased in the above way, are in a favorable direction from the viewpoint of simple storage of liquids, but rather involve problems in the case of consideration of distribution and diffusion of the liquids under circumstances of actual use of diapers. The large amount of water-absorbent resin swells to become a soft gel due to water absorption to cause a phenomenon which is called “gel-blocking” that much hinders the permeation and diffusion of the liquids. Known examples of arts of improving these liquid permeability and liquid diffusibility include the following arts.
There is known a method in which there is used a hydrogel-formable absorbent polymer of which: the saline flow conductivity (SFC) value is at least about 30 (10−7·cm3·s·g−1); the capacity value of performance under pressure (PUP) is at least 23 g/g under a closing pressure of 0.7 psi (5 kPa); and the basis weight is at least about 10 gsm (patent document 1).
There is known an absorbent structure of which: the water-absorbent resin concentration is at least 40 mass %; the saline flow conductivity (SFC) value is at least about 30 (10−7·cm3·s·g−1); and the capacity value of performance under pressure (PUP) is at least 23 g/g under a closing pressure of 0.7 psi (5 kPa) (patent document 2).
There is known a method in which: in an absorbent structure, a water-absorbent resin of which the gel layer permeability (GLP) is at least 4 (10−7·g−1) is used for an upper layer, and a water-absorbent resin of which the absorption capacity under load (AAP) is at least 15 (g/g) under a load of 50 g/cm2 is used for a lower layer (patent document 3).
There is known a method in which a polycation is covalently bonded to a water-absorbent resin (patent document 4).
There is known an absorbent material including a mixture of: a plurality of absorbent gel-formable particles including a water-insoluble and water-swellable polymer; and an absorbency-improving polymer which is reactable with at least one component contained in urine (patent document 5).
There is known a method in which there is used a mixture of a spherical water-absorbent resin and a non-spherical water-absorbent resin (patent document 6).
There is known a method in which there is used a water-absorbent resin of which the saline flow conductivity (SFC) value is at least 5 (10−7·cm3·s·g−1) and which contains a permeability-keeping agent (patent document 7).
There is known an absorbent structure having a division surrounded by a continuous area of a hydrogel absorbent polymer (patent document 8).
There is known a hydrogel-formable absorbent polymer of which: the dynamic gelation rate is at least about 0.18 g/g/sec; and the ability value of performance under pressure (PUP) is at least about 25 g/g under a binding pressure of 0.7 psi (5 kPa); wherein the hydrogel-formable absorbent polymer has a mass-median particle size of at least about 100 μm when the hydrogel-formable absorbent polymer exists in the form of particles (patent document 9).
There is known a water-absorbent material, in the rear half portion of which there is placed 55 to 100%, favorably 60 to 90%, of the entire mass of an absorbent gelling material (patent document 10).
There is known an absorbent member having a liquid-acquiring zone (patent document 11).
There is known a water-absorbent resin of which: the absorption capacity under load is not less than 30 g/g; and the gel layer liquid permeation rate is not more than 100 seconds (patent document 12).
There is known a process including the step of grinding crosslink-polymerized particles until their bulk density increases to not less than 0.72 g/ml (patent document 13).
There is known a process in which a water-absorbent resin is surface-treated with a surface-treating agent including a polyol and a cation (patent document 14).
There is known a process in which a water-absorbent resin is surface-treated with a surface-treating agent including an organic crosslinking compound (except polyols) and a cation (patent document 15).
There is known a water-swellable polymer as crosslinked with an unsaturated amino alcohol (patent document 16).
There is known a hydrogel-formable polymer of which: the saline flow conductivity (SFC) is at least 40 (10−7·cm3·s·g−1); the AUL is at least 20 g/g under 0.7 psi (4826.5 Pa); and the Frangibility Index (FI) is at least 60% (patent document 17).
There is known a water-insoluble and water-swellable hydrogel which is coated with steric or electrostatic spacers and of which: the AUL is at least 20 g/g under 0.7 psi; and the gel strength is at least 1,600 Pa (patent document 18).
However, in cases where the above prior water-absorbent resins as disclosed in patent documents 1 to 18 have high liquid permeability, spaces between swollen gel particles increase to bring about deterioration of the capillary suction force. The deterioration of the capillary suction force is a cause that a residual liquid remaining not taken into the water-absorbent resin increases on surface layers of the sanitary materials to thus bring about: deterioration of the dry-touch property; an unpleasant feeling during wearing; and skin diseases such as skin eruption. In order to avoid such problems to maintain absorption properties of the absorbent structure, the ratios of the hydrophilic fibers and the water-absorbent resin are axiomatically limited, so a limit occurs also to the thinning of the sanitary materials.
That is to say, in the prior arts, the liquid permeability is pursued, but, to the capillary suction force as lost thereby, there has been paid no attention. In addition, in the prior arts, though the particle diameter distribution is a very important factor for the liquid permeation and the capillary suction force, yet, about it, there has been made no detailed explanation of the relationship of the particle diameter distribution with the liquid permeability and the capillary suction force. As matters now stand, almost no detailed explanation is made particularly also about a particle diameter distribution which is excellent in both of the liquid permeability and the capillary suction force or about a means for achieving such a particle diameter distribution.
(2) Particle Diameters:
In addition, such as the following arts are known as examples of water-absorbent resins having a controlled particle diameter distribution and arts in which particle diameter distributions of water-absorbent resins are controlled.
There is known an absorbent article including a high-molecular gelling agent having a mass-median particle diameter of 400 to 700 μm (patent document 19).
There is known a hydrogel polymer of which: the average particle diameter is in the range of 100 to 600 μm; and the logarithmic standard deviation σζ of the particle diameter distribution is not more than 0.35 (patent document 20).
There are known polymer material particles which are water-insoluble, absorbent, and formable into a hydrogel, and of which at least 80% are particles of such a size as passes through a sieve having a mesh opening size of 297 μm and is captured on a sieve having a mesh opening size of 105 μm (patent document 21).
There are known water-absorbent resin particles which have a specific surface area of 50 to 450 m2/g and contain a fine powder of hydrophilic silicon dioxide having a hydrophilicity of not less than 70% (patent document 22).
However, as to the above arts cited in such as patent documents 19 to 22, none of them is an art for achieving a particle diameter distribution specified for obtaining a water-absorbing agent which is excellent in both of the liquid permeability and the capillary suction force. In addition, the ranges of the resultant particle diameter distributions are also broad, and the absorption capacity without load and the absorption capacity under load are also different. Therefore, it has been difficult to obtain from the above arts the water-absorbing agent which is excellent in both of the liquid permeability and the capillary suction force.
[Patent document 1] WO 95/26209
[Patent document 2] EP 0951913
[Patent document 3] EP 0640330
[Patent document 4] WO 97/12575
[Patent document 5] WO 95/22356
[Patent document 6] WO 98/06364
[Patent document 7] WO 2001/066056
[Patent document 8] WO 97/25013
[Patent document 9] WO 98/47454
[Patent document 10] WO 96/01608
[Patent document 11] WO 97/34558
[Patent document 12] JP-A-089527/2001 (Kokai)
[Patent document 13] EP 1029886
[Patent document 14] WO 2000/53644
[Patent document 15] WO 2000/53664
[Patent document 16] U.S. Pat. No. 6,087,450
[Patent document 17] U.S. Pat. No. 6,414,214
[Patent document 18] US 2002/128618
[Patent document 19] U.S. Pat. No. 5,051,259
[Patent document 20] EP 0349240
[Patent document 21] EP 0579764
[Patent document 22] EP 0629411
As to the aforementioned prior art water-absorbent resins and/or water-absorbing agents of such as patent documents 1 to 22, the liquid permeability is improved, but, at the same time, the performance deterioration such as capillary suction force deterioration is caused. Thereby, the diffusibility and the liquid permeability in a water-absorbent structure which is a component material for such as sanitary materials are improved, but, on the other hand, there is caused the performance deterioration such that the dryness property and the liquid-retaining ability are deteriorated. Therefore, the aforementioned prior art ones have not necessarily been satisfactory ones. That is to say, there have occurred problems such that, even if either one of the performances is enhanced, the other performance is deteriorated. In order to solve such problems, it has been expected that there appears a water-absorbing agent which combines both performances of the liquid permeability and the capillary suction force.